Factor VIII-related antigen (F.VIII R:Ag) has been found on and within normal platelets. This pool of F.VIII R:Ag is intrinsic to the platelet and non-exchangeable with circulating plasma F.VIII R:Ag. Any interaction of this platelet F.VIII R:Ag with plasma coagulation factors is merely speculative at this time. This project seeks to quantitate the amount of F.VIII R:Ag located on/within the platelet and identify the site(s) at which it is located and any role it may play during platelet activation and plasma coagulation activation. In addition, by using this system of anti-F.VIII R:Ag/F.VIII R:Ag as a probe/marker, we hope to clarify the many processes involved in platelet activation and better understand the energy-dependent nature of these processes.